The Combustion Chamber is the heart and soul of any spud gun. It is responsible for generating the pressure needed to propel the spud down the barrel. As such there are some basic things we need to do to get the combustion chamber ready for this purpose.
Pressure Sensing
Obviously most people are not going to add a pressure sensor to their spud gun build. We are, because we are geeks, and the whole point of this project is to try and demonstrate Pressure/Time curves. For the pressure sensor we are using an automotive oil/gas pressure sensor rated for 500 PSI.
This pressure sensor uses a pressure sensitive resistor and contains its own amplifier circuit. Input voltage is 5 Volts and the output voltage is variable dependant on pressure .5 thru 4.5 volts. 1 PSI is roughly .008 volts or 8 millivolts.
Typically pressure readings for ballistics are done at 200,000khz, or 200,000 readings per second. These sample rates are far faster then what can be done with an Arduino, or Beaglebone. However it is well within the range of even the most basic of oscilloscopes.
The signal resolution for this inexpensive O-Scope is 12 bit, so our pressure resolution is 500/4096 or .12psi. This is more than enough for this project. The down side to this O-Scope is that I cannot port the data directly to the computer. I’ll be able to see the pressure curve, and get the peak pressure from the scope, but I will need to take a picture of the screen in order to share it. For proof of concept this will work. We will get fancier down the road.
Installing the sensor is pretty straight forward. The sensor has a 1/8 NPT thread, and we drilled a hole in the front of the chamber and tapped it for 1/8 NPT.
We positioned the sensor closest to the barrel. Since this is a large chamber, it is conceivable that there is a pressure gradient within the chamber with higher pressures occurring near the point of ignition then what we see at the base of the projectile. We are most interested in the pressure that is seen by the projectile, and by locating the sensor near the breech of the barrel we are able to see this pressure.
Installing the Fan
In order to ensure uniform mixture and ignition for the spud gun we need some sort of fan to “Stir” the air within the cannon. It is important that the fan is not the source of ignition so it must be a brushless fan. Luckily these fans are a dime a dozen, used in computers and electronics for cooling.
For a 4in combustion chamber a 120mm case fan works well, but requires some trimming in order for it to fit. There are two wire, three wire, and four wire fans out there. Typically with all these designs only two wires are needed. The other wires provide information back to the computer about fan RPM and Temperature. On our fan, the Yellow and Black wire was V+ and V-, this may be different for your fans.
Typical power supplies for these fans are 12v, but they are like any other Brushless DC motors RPMs are determined by voltage supplied. Thus they will run on higher or lower voltages, the RPM will just be different.
A 120mm fan will fit within the threaded cleanout trap if you trim the corners. I did it with a dremel. It makes a bit of a mess, so be warned. When screwed in the cleanout trap will hold the fan in place. The fan will not circulate the air front to back, it will only get the air spinning within the chamber. Natural turbulence within the air mass will help mix the fuel so that it is spread evenly throughout the chamber prior to ignition.
After firing, we can unscrew the trap and open the rear of the cannon. This will allow the fan to draw in fresh air into the chamber, and expel the remaining exhaust gases out the barrel. With this being a breech loading model, we can be reasonably sure that each new shot will have a full charge of fresh air.
In order to provide power to the fan we need to insert electrical terminals for the fan. This is done by drilling #7 holes through the coupler, threading the holes for 1/4-20 and then screwing in some studs that will allow for electrical connections to be made.
Ignition Terminals
For the ignitiion system we will be using a high voltage generator. These are inexpensive and can be had on Amazon for about $7.
Traditionally spud guns use Barbecue lighters, or other piezoelectric devices in order to create a spark. We are going more of the high tech route. Will will be making an ignition strip that mounts on the inside of the combustion chamber and powered by the high voltage generator. The trigger will be fed through an Arduino, which will switch on the generator via relay for a preprogrammed amount of time.
This will give us complete control over some of the variables that might be encountered when building a spud gun.
The Fuel Port
For the first few tests we plan on using propane as a primary fuel. This fuel will be metered into the spud gun. The stoichiometric (fuel to air) ratio for propane is ~4.1% by volume. We will do a more detailed write up on building the fuel metering system, right now we just need to do the port as to where the fuel will be introduced.
I chose to introduce the fuel right in front of the fan. As I stated before, the fan will not move the air front to back as it has to place to draw the air from and push the air to. What it will do is get the air spinning within the chamber. The spinning of the air creates turbulence and will help the fuel to mix evenly throughout the chamber. It may not matter where I introduce the fuel into the chamber, but I suspect the most turbulent air will be right in front of the fan.
For now the fuel port is a 1/8 NPT threaded hole in the side of the combustion chamber.
Completion
With the hole drilled and threaded for the fuel port the combustion chamber is completed. We will add the wiring and the ignition strip to the chamber in a separate write up.
There are alot of spud guns that do not use a fan, or use only a simple BBQ igniter for the ignition. Obviously we are taking our time and making this build a little more complex in the hopes we get more consistent results. I remember being a teen and reading what others were doing back in the day. People would purchase cheap stun guns and tear out the guts to use for their ignitor.
Between the availability of cheap programmable microcontrollers, high voltage generators, and a whole host of other inexpensive electronics we can build exactly what we want from the ground up without repurposing items. It is really the golden age of Makers.